Cut-off waveguide attenuator



April 12, 1955 M. B. HALL CUT-OFF WAVEGUIDE ATTENUATOR Filed May 3, 1946 INVENTOR. MAURICE B. HALL ATTORNEY United States Patent CUT-OFF WAVEGUIDE ATTENUATOR Maurice B. Hall, Wilmington, Del., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application May 3, 1946, Serial No. 666,879

9 2 Claims. (Cl. 333-81) This invention relates in general to attenuators and in particular to radio frequency power attenuators.

This invention is applicable to those radio frequency power attenuators which employ a length of waveguidebelow-cutoff as the attenuating element. This element is a length of waveguide having dimensions smaller than the minimum required dimensions for transmission of a chosen frequency and thus this length of waveguide-below-cutoff will attenuate the radio frequency power when such power is passed through the element. These attenuators are usually constructed of circular metallic tubing energized at one end from a coaxial line with loop coupling into the tubing, and energize a section of normal sized waveguide from loop coupling at the other end of the tubing. The amount of attenuation is varied by effectively varying the length of waveguide-below-cutoif by varying the insertion depth of the coaxial feed coupling loop. Theoretically the attenuation in decibels varies linearly with the length of waveguide-beloyv-cutoff.

In the past these attenuators have had undesirable high insertion losses and have not had a linear attenuation in decibels. Two reasons for these undesirable features are: one, the coupling occurred through exciting modes other than the desired mode because the feed loop was formed by simply bending the center conductor of the coaxial feedline sharply and soldering its end to the outer conductor and as the end of such a loop is along a radius of the attenuating tube it excited the TMo,1, or coaxially symmetrical mode as readily as the desired TE1,1, or transmission mode; and two, reflection occurred at the junction between the cutoff tube and the transmission lines due to improper matching of impedances. These previous attenuators also employed sliding electrical contact between the movable and stationary parts of the attenuator which caused wearing of the cutoff tube and consequently changed the calibration of the attenuator.

It is the primary object of this invention to minimize those undesirable features in radio frequency attenuators.

It is a further object of this invention to provide new and improved coupling loops to discriminate in favor of the desired operating mode.

It is a further object of this invention to provide an improved waveguide-below-cutoif attenuator which has low insertion loss and which has minimum reflections from the junctions of the cutoff tube with the adjoining transmission lines.

It is a still further object of this invention to eliminate the sliding electrical contacts between the movable and stationary parts of the attenuator.

Other and further objects of this invention will be apparent from the following specifications when taken with the accompanying drawing in which:

Fig. 1 is an axial sectional view of an attenuator showing features of this invention, and

Fig. 2 is a sectional view of the attenuator taken along line II-II of Fig. 1, showing other features of this invention.

The embodiment of the invention shown in Fig. 1 is a sectional view through the axis of waveguide-belowcutoff tube 14, the coaxial feed line, and waveguide 15. The coaxial feed line is composed of metallic tubing 10, insulation 11, and center conductor 12. The feed loop 13 is formed by bending center conductor 12 in a rectangular shape, as shown, and soldering or otherwise fastening the end to outer conductor 10. Loop 13 is made rectangular as shown in Fig. 1 so that the radiating 'por- 2,706,276 Patented Apr. 12, 1955 Ice tion of the loop is across an entire diameter of attenuator tube 14. Such a loop discriminates strongly in favor of the desired TE1,1 or transmission mode. This loop is also made long enough so that any radiation into attenuator tube 14 from any but the most extended part of the loop will be attenuated to insignificance by the time that radiation reaches the most extended part of the loop. Loop 13 also acts as an impedance matching transformer between waveguide-below-cutoif 14 and the coaxial line through which the energy is incident, and, by controlling the dimensions of loop 13, any reflections resulting from the discontinuity in the impedance occurring at the transition from the coaxial line to the waveguidebeIow-cutolf can be reduced to negligible proportions.

To reduce any reflections that may occur at the transition from waveguide-below-cutoif 14 to waveguide 15, the impedance of waveguide-below-cutoif 1 4 is matched to the impedance of waveguide 15 by coupling through inductive iris 16. The combination of rectangular feed loop and inductive iris coupling reduces insertion losses and makes the attenuation versus position of loop curve more nearly approach the theoretical straight line. Wear on the cutoff tube is minimized by using a series of reactive chokes 17 followed by bearing 18 made of insulating material and absorbing ring 19, the latter being included to prevent leakage of energy that might otherwise by-pass the cutoff tube. Absorbing ring 19 can be of material such as Polyiron which is a mixture of plastic binder and powdered iron. The reactive chokes alone provide a better electrical circuit than sliding contacts, and bearing 18, whose function is merely to keep the assembly concentric in tube 14, is constructed of softer material than tube 14 to prevent wear on the tube.

This invention is not to be limited to the details of the foregoing specification but is to be limited only by the following claims.

What is claimed is:

1. In combination a section of hollow circular wave guide having a predetermined cut-off frequency, a coaxial line having inner and outer conductors, one end of said coaxial line being telescopically fitted within one end of said circular wave guide and arranged for axial movement therewithin, said one end of said coaxial line being terminated in a substantially rectangular coupling loop connected between the inner and outer conductors of said coaxial line, a source of electromagnetic energy of a frequency below the cut-off frequency of said circular wave guide coupled to the other end of said coaxial line, said coupling loop comprising a first linear portion extending angularly from said inner conductor to a point immediately adjacent an inner wall portion of said wave guide, a second linear portion extending axially from said point a predetermined distance along said guide to a second point, a third linear portion extending diametrically across said guide from said second point to a third point immediately adjacent and opposite inner wall portion of said wave guide and a fourth linear portion extending axially from said third point to the outer conductor of said coaxial line, the lengths of said second and fourth portions and their proximity to adjacent inner wall portions of said wave guide being such that the electromagnetic energy radiated therefrom is attenuated to an insignificant amount by the time it reaches that portion of the wave guide whereat the third linear portion is located whereby substantially all of the electromagnetic energy propagated in said guide emanates from said third linear portion of said coupling loop 2. An electromagnetic energy attenuator comprising, in combination, a section of hollow circular wave guide having a predetermined cut-off frequency, a coaxial line having inner and outer conductors, one end of said line being telescopically disposed within one end of said wave guide, a source of electromagnetic energy of a frequency below the cut-off frequency of said wave guide coupled to the other end of said coaxial line, said one end of said coaxial line being terminated in a substantially rectangular coupling loop connected between the inner and outer conductors of said coaxial line, said coupling loop including a first linear portion extending angularly from said inner conductor to a first point immediately adjacent an inner wall portion of said wave guide,

a second linear portion extending axially from said first point a predetermined distance along said wave guide to a second point, a third linear portion extending diametrically across said guide from said second point to a third point immediately adjacent an inner wall portion of said wave guide and a fourth linear portion extending axially from said third point to the outer conductor of said coaxial line, the lengths of said second and fourth portions and their proximity to adjacent inner wall portions of said guide being such that electromagnetic energy radiated from these portions is attenuated appreciably by the time it appears at that portion of the wave guide whereat the third linear portion is located whereby substantially all of the electromagnetic energy propagated in said wave guide emanates from said third linear portion, an output wave guide joined to the other end of said hollow circular wave guide, said output wave guide having a cut-oif frequency below the frequency of said source of electromagnetic energy and a reactive iris positioned at the juncture of said output wave guide and said hollow wave guide.

References Cited in the file of this patent UNITED STATES PATENTS King Mar. 21, King Apr. 16, King Feb. 18, Ginzton Sept. 10, Sloan Nov. 19, Ramo Nov. 26, Linder Sept. 30, Salisbury Oct. 19, Fernsler Mar. 29, Van Beuren June 28, Hegbar Nov. 22, Hansen July 11, Latimer Oct. 10, Jaynes June 19, 

